The present invention relates generally to the halftoning of images. More particularly, the present invention relates to a method of and system for rendering a halftone by utilizing a pixel-by-pixel comparison of the gray scale image against a blue noise mask.
Many printing devices are not capable of reproducing gray scale images because they are bi-level. As a result, the binary representation of a gray scale image is a necessity in a wide range of applications such as laser printers, facsimile machines, lithography (newspaper printing), liquid crystal displays and plasma panels. Gray scale images are typically converted to binary images using halftone techniques. Halftoning renders the illusion of various shades of gray by using only two levels, black and white, and can be implemented either digitally (facsimile machines, laser printers) or optically (newspaper printing).
Halftoning algorithms are classified into point and neighborhood algorithms according to the number of points from the input gray scale image required to calculate one output point in the output binary image. In the case of digital halftoning, points correspond to pixels. In point algorithms, the halftoning is accomplished by a simple pointwise comparison of the gray scale image against a nonimage, usually aperiodic (but not always) array or mask. For every point in the input image, depending on which point value (the gray scale image or the mask) is larger, either a 1 or 0 is placed respectively at the corresponding location in the binary output image. Halftoning using neighborhood algorithms is not done by simple pointwise comparison, but usually requires filtering operations that involve a number of points from the input gray scale image in order to calculate one point in the output image.
At present, given the existing halftoning algorithms, the choice for a specific halftoning algorithm depends on the target device and always requires a trade-off between image quality and speed. Neighborhood halftoning algorithms result in a good image quality (although the image is not completely artifact free), but they are slow and cannot be optically implemented. That leaves point algorithms as the only choice for optical applications such as newspaper printing. Point algorithms are fast and are suitable for all target devices, but the output usually suffers from artifacts such as periodic artifacts and false contours.
The halftoning system disclosed herein utilizes a point-algorithm, and combines the output image quality of neighborhood algorithms with the speed and wide application range of point algorithms. A point algorithm is utilized and the halftoning is achieved by a pixelwise comparison against a nonimage array, called the "blue noise" mask.
The digital halftoning of images with multiple levels, such as gray scale levels, is known in the art. Two major techniques are currently in use. They are the ordered dither and the error diffusion methods. See Digital Halftoning by R. Ulichney, MIT Press, Cambridge, Mass. (1987). See also R. W. Floyd and L. Steinberg, "Adaptive algorithm for spatial gray scale", SID International Symposium Digest of Technical Papers, pps. 36-37. The Floyd and Steinberg paper is directed to the digital halftoning of a gray scale.
The major ordered dither techniques are the clustered-dot dither and dispersed-dot dither techniques. A white noise random dither technique is seldom utilized because it produces the poorest quality image and, of the other two dither techniques, clustered-dot is by far the most used. Both of those techniques are based upon a threshold screen pattern that is of a fixed size. For example, 6.times.6 threshold screens may be compared with the digital input values. If the input digital value is greater than the screen pattern number, a 1 is produced and, if it is less, a 0 value is assigned. The number of levels that can be represented using either technique depends on the size of the screen. For example, a 6.times.6 screen can produce 36 unique levels.
More levels can be achieved with larger patterns, however, a reduction in the effective resolution occurs because the ability to transition among levels is at a coarser pitch. At the pixel rate of about 300 to 500 per inch, which is the average pixel rate of copiers and laser printers, the pattern artifacts are visible for screen patterns larger than 4.times.4, and, since 16 levels do not provide an adequate precision for typical continuous-tone imagery, a suboptimal resolution is usually obtained.
One solution to such a problem is disclosed by Ulichney in a paper "Dithering with Blue Noise" published in the Proceedings of the IEEE, Vol. 76, No. 1, January 1988. In that article, a method of spatial dithering is described which renders the illusion of continuous-tone pictures on displays that are capable of only producing binary picture elements. The method produces a blue noise pattern high frequency white noise from a filter to provide desirable properties for halftoning. More specifically, Ulichney uses perturbed-weight error diffusion methods which when digitally implemented run at a much slower speed (approximately 100 times slower) than is attainable with the present invention.
Error diffusion techniques, such as that disclosed in the Ulichney IEEE article, are fundamentally different from ordered dither techniques in that there is no fixed screen pattern. Rather, a recursive algorithm is used that attempts to correct errors made by representing the continuous signal by binary values.
The error diffusion method described by Ulichney, and others, such as Floyd and Steinberg, also has the disadvantage that it requires scanning, convolution-style calculations and, although it can be implemented for use with copiers, facsimile machines, etc., requires local calculations. It cannot, however, be optically implemented. In addition, all error diffusion techniques, including those described by Ulichney and Floyd and Steinberg, show scanning and start-up artifacts, which are not present in the instant invention. Also, while Ulichney describes a method that produces blue noise, the blue noise patterns produced by the present invention are more isotropic than those produced by Ulichney or other error diffusion methods. Utilizing ordered dither methods produces notably periodic patterns that are even much more obtrusive than those produced by error diffusion methods.
In some prior art systems, all dot profiles corresponding to different gray levels were derived independently, as if each grade level was its own special case. Thus, for example, in U.S. Pat. No. 4,920,501, to Sullivan et al., many individual dot profiles, corresponding to the desired number of gray levels, must be stored. In the present invention, on the other hand, dot profiles are built "on top of" the profile from lower gray levels, such that a single valued 2-dimensional function, that is, the cumulative array or blue noise mask, can be constructed. When that single valued function is thresholded at any level, the resulting binary pattern is exactly the blue noise dot profile design for that particular gray level, p(i,j,g), where p can be one or zero corresponding to black or white, i and j are coordinates of pixels, and g represents a gray level 0&lt;g&lt;1.
Another drawback to prior art methods is that the dot profile for a given gray level was designed to have blue noise properties by indirect methods, such as using an error diffusion filter with perturbed weights (Ulichney) or by a "simulated annealing" algorithm, as in U.S. Pat. No. 4,920,501. The method disclosed herein is advantageous with respect to the prior art in that the desired blue noise power spectrum is produced through the use of a filter on the dot profile and is implemented directly in the transform domain. Such filtering results in a nearly ideal blue noise pattern with implicit long-scale periodicity because of the circular convolution implicit in the use of discrete Fourier transforms. However, the filtered pattern is no longer binary. Thus, a minimization of error approach is utilized in which the largest differences between the ideal, filtered, blue noise pattern and the unfiltered dot profile are identified. The magnitude and location of those differences indicate the pixels in which ones and zeros could be changed to produce a more ideal blue noise dot profile.
Display devices, including printing devices as well as media, have their own unique input-output characteristics. In some uses, such as medical ultrasound imaging, the user has traditionally been provided with some control as to the final gray scale mapping. For example, the user may be able to select between high and low contrast images. The display and film characteristics, however, must be accounted for in each rendition.
In the area of halftone rendering, traditional halftone screens using small (for example, 8.times.8 pixel) kernels provide only limited degrees of freedom to alter the input-output characteristics and usually a linear cumulative distribution function (CDF) has been reported. See Digital Halftoning by R. Ulichney, MIT Press, Cambridge, Mass. (1987). See also, R. Baker, "An Optimum Method for 2 Level Rendition of Continuous Tone Pictures", IEEE International Conf. Comm., 1973, and G. C. Reid, Postscript Language Program Design (green book), Addison-Wesley Publishing Co., New York, N.Y., (1988), page 137. By a linear CDF, it is meant that 10% of the halftone kernel pixel contents will be less than 10% of the maximum value and that 50% of the pixels will contain values less than 50% of the maximum values, and so forth.
In the case of the blue noise mask method disclosed herein, a large unstructured pattern of, for example, 256.times.256 pixel kernels, provides sufficient degrees of freedom to modify the cumulative distribution function so as to provide both linear and non-linear mappings of input and output characteristics. That makes it possible to construct specialized blue noise masks in which a particular printer output and media characteristics can be compensated for by a modified blue noise mask generated as disclosed in this application.
The present inventive method herein may also be applied to color halftoning, by independently thresholding each of the component colors against the disclosed blue noise mask and then overprinting. Such method produces a pleasing pattern without any blurring of the image. Such method is a great improvement over the known prior art, which is discussed below.
In U.S. Pat. No. 5,010,398, there is disclosed a method for color corrections by dry dye etching using a photographically produced mask which may be used in the production of printing plates for printing reproductions of colored originals and in which a contact print is overexposed to a photographic mask. The photographic mask is constituted so as to isolate a selected area in addition to being exposed normally for obtaining an exact copy of an original halftone separation. The mask is electronically generated by scanning each separation, digitizing each signal and then storing the digital values in a digital storage device.
U.S. Pat. No. 4,974,067 relates to a multi-step digital color image reproducing method and apparatus which separates an original image into a plurality of color components to produce image data associated with each respective one of the color components. The image data are individually processed to provide record color component density data which data are used to record a halftone representation pattern of that color component.
An apparatus and methods for digital halftoning is disclosed in U.S. Pat. No. 4,924,301 for producing halftone screens or color separations from continuous tone intensity signals that are supplied by an optical scanner. Using a digital signal processor, the continuous tone intensity values are processed to establish memory maps which, in conjunction with a digital data output device such as a laser printer, produces the desired halftone screen. The digital signal processor utilizes a dither matrix in order to produce halftone screens having a screen angle that does not substantially differ from the screen angles of the yellow, cyan and magenta color separations in conventional four color halftone printing. A dither array is also utilized to produce the halftone screens having a screen angle that substantially differs from the screen angle used in the black halftone color separation in conventional four color halftone printing.
U.S. Pat. No. 4,342,046 relates to a contact screen for making color separation halftone blocks for use in a picture reproducing machine in which a plurality of halftone screens having different screen angles are arranged on a base film in the corresponding positions of color separation reproduction pictures to be reproduced on the base film and transparent blank spaces are formed between two adjacent halftone screens.
A method and apparatus for making monochrome facsimiles of color images on color displays is disclosed in U.S. Pat. No. 4,308,533 for making 35 MM color slides from a color image created on a color cathode tube terminal. U.S. Pat. No. 3,085,878 relates to the preparation of traditional halftone screens for color separation.
U.S. Pat. No. 4,657,831 relates to the production of electrophotographic color proofs of halftone dot pattern images which closely simulate the dot gain of prints made with lithographic plates and liquid inks.
A process for the production of photographic masks is disclosed in U.S. Pat. No. 4,997,733 in which such masks are used for the tonal correction by dry dot etching in which the selection of a particular halftone color separation image or overlaying registering combination of halftone color separation images is determined on the basis of optical density differences in at least one such halftone color separation. Such differences include differences in contrast, between each area to be isolated as a substantially transparent area, and at least one particular background area surrounding each area to be isolated.
U.S. Pat. No. 4,477,833 is directed to a method of color conversion with improved interpolation in which an apparatus for converting a color image from one colored space to another colored space includes a memory which stores a finite number of output signals which represent colors in the output space and which is addressed by signals representing a color in the input space. The interpolation process is utilized in order to derive an output color value for an input color located between colors stored in the memory.
A method and apparatus for producing halftone printing forms with rotated screens based upon randomly selected screen threshold values is disclosed in U.S. Pat. No. 4,700,235. The screens have arbitrary screen angles and screen width. Screen dots are exposed on a recording media by means of a recording element whose exposure beam is switched on and off by a control signal.
None of the foregoing references have the advantages of the use of the blue noise mask method disclosed herein in producing pleasing, isotropic, non-clumpy, moire resistant patterns with only some spreading out of the color or ink but with no blurring of the image.